1. Field of the Invention
The present circuits and methods relate generally to the field of the control of electric machines, and more particularly to circuit configurations for and methods of controlling permanent magnet synchronous motors.
2. Background of the Invention
To have good dynamic response for a permanent magnet synchronous motor (PMSM), the controller should be able to control the torque component of the inverter output current to the motor based on the rotor position. The off-shelf hybrid integrated circuit (IC) chip ML4425 (from Micro Linear Corporation, San Jose, Calif.) provides an example of position sensorless control for brushless DC motors based on back electromotive force (back EMF) estimation. FIGS. 1A and 1B are adapted from an extract from the May 1997 ML4425 technical specification which illustrates a typical prior art application using an IC chip 10, such as the ML4425 chip. However, IC chips, such as the ML4425 chip, have limitations for wide speed range PMSM control systems. Such limitations include, for example, that alternating current (AC) motor current is not controllable, and the use of a single control loop structure in which only one of the two control modes—current control or speed control—can be selected.
A current limit control is incorporated in the IC chips, such as the ML4425 chip, and direct current (DC current) is used for the control. As is known, in inverter motor drive applications, the motor torque is proportional to its torque component of AC phase current, while motor power, which is shaft torque multiplied by shaft speed, is proportional to DC current with a constant DC bus voltage. Consequently, for a motor at low speed with high shaft torque, the DC current will be low, while the AC current is high due to low AC voltage. Thus, the DC current cannot be used to control motor torque in the entire speed range, and the DC current limit control used in IC chips such as the ML4425 chip cannot provide effective AC over current protection for AC permanent magnet motors and the devices of the inverter when the motor runs at low speed.
FIG. 2 is a block diagram which illustrates an example of a typical motor control system configuration utilizing an IC chip 10, such as the off-shelf ML4425 chip shown in FIGS. 1A and 1B. Referring to FIG. 2, the configuration includes inverter 12, which is an insulated gate bipolar transistor (IGBT) based component that converts DC voltage into AC with a variable frequency, and logic control and gate drive 14 which is a proprietary component built specifically for use with the particular IC chip 10. IC chip 10 is not intelligent but is simply an IC chip that controls a brushless DC motor. IC chip 10 provides gate drive control signals which control the IGBTs in inverter 12. It also includes a back EMF sampler or sensor 16, as shown in FIG. 1B, the input to which are voltage signals 152, 153, 154, as shown in FIG. 2. Basically, IC chip 10 of FIGS. 1A and 1B estimates the back EMF of a motor 18, such as a brushless DC motor, and in that way, it can estimate the motor speed. IC chip 10 combines back EMF sensor 16, voltage control oscillator (VCO) 20, and a sequencer to form a phase-locked loop which provides a speed sensorless control for motor 18. An advantage of the ML4425 chip is that it realizes a kind of sensorless control performance by use of back EMF estimation without discrete speed or position sensing elements.
Referring further to FIG. 2, in the illustrated configuration, another feedback is required, which is provided via resistor Rdc 22. Basically, an attempt is made to measure the DC current. The DC voltage is fixed, and an attempt is made to control the DC current within certain limits. The illustrated configuration limits the power output, which brings up the limitations of IC chip 10. For example, IC chip 10 can only limit the DC current, which means it can only limit the power. When an attempt is made to start motor 24, motor 24 is at a very low speed, and the power is very low, because the output voltage is very low and the power is the product of the AC voltage and current. However, since the rated motor power is relatively high, such as 30 kilowatts, it is necessary to fix the DC current limit to a relatively high value. A problem occurs if a high DC current limit is selected, AC current cannot be limited at low speed because the power is equal to the AC current multiplied by the AC voltage, and this power is also proportionate to the speed. At low speeds the power is very low, but the current can be very high. The problem here is that the AC current is very high, and it cannot be limited. Thus, quite often when an attempt is made to start motor 24, because the AC current is too high, it simply trips the circuit protection and prevents the successful starting of motor 24.
Referring again to FIG. 2, another problem in the illustrated configuration is that only power is regulated. When the power for the DC current is higher than the limit, it stops the PWM switching. Basically, the configuration attempts to regulate power. The configuration has only one of the two control modes. If current control is used, the speed regulation will not be available. In current control mode, it only limits the DC current. When the speed loop is used, the current loop must be disconnected, which is another limitation of IC chip 10. In addition, the DC current required by IC chips such as the ML4425 chip must be obtained through a serial resistor at a DC bus of the inverter, and it is difficult to provide isolation in high power inverter systems with a high DC bus voltage. There is a current need for an improved configuration that overcomes thee limitations of IC chip 10.